The present invention relates to a hydraulic system for synchronized extension of two or more cylinders. For example, the present invention is useful on a lift table where table surface must be raised and/or lowered while maintaining levelness, despite non-uniform loads. However, the present apparatus is not believed to be limited to only this particular application, since distribution of identical amounts of hydraulic fluid can be used very effectively in many different applications. Also, the present invention includes additional aspects, including an automatic resynchronization sequence, a filling sequence without the need to draw, bleed, or to evacuate hydraulic lines, and an air purge sequence also without the need to draw a vacuum or bleed hydraulic lines.
Many attempts have been made to synchronize hydraulic systems in the past. Generally these synchronizing systems use multiple gear pumps on a common shaft, one for each cylinder, or special proportioning valves, or other means in an attempt to deliver an identical amount of hydraulic oil to each cylinder. None of these systems are completely successful because loss of oil in the various devices accumulate and adversely affect synchronization. For example, the gear units have losses around the sides of the gears and through the gear tooth surfaces. The systems using proportioning valves also experience oil loss because of the clearance between the valve body and the spool. Oil leaks and entrapped air and non-uniform loading also adversely affect synchronization and cause dissimilar extension of cylinders.
The loss of oil in any individual cylinder circuit especially hinders the functionality of the multi-cylinder system to move or lift objects in the intended even manner. Generally the loss of oil is a function of a number of operating cycles and the load applied to the cylinders. The worst case is demonstrated when the load is not evenly distributed between all of the cylinders being used. If a higher percentage of the load is assigned to one of the cylinders, then the leakage found in that cylinder circuit will be greater in volume than the leakage in the rest of the circuits. Over time, the higher leakage in one of the cylinder systems will cause the lifting cylinders to go out of phase and subsequently cause the system to fail. Also, many synchronized hydraulic systems that use multiple cylinders in parallel will bind and cause stress concentrations leading to premature wear and increased maintenance.
Resynchronization and line-purging to eliminate trapped air in known synchronized hydraulic systems is undesirably time-consuming and labor-intensive, and is difficult to accomplish without messy maintenance procedures such as disconnecting, bleeding, and reconnecting hydraulic lines. Further, repeated disconnections and re-connections undesirably increase the risk of new leaks. There are many situations when it is very desirable to use two cylinders to move an object. Sometimes more force is required than can be developed with one cylinder. In other cases the object is rectangular such as a table, or a press ram, or a slide of some sort. In most cases these items are wide enough to be unstable if operated by one center mount cylinder. In order to use one center mount cylinder very heavy bearing guides must be provided at the outer edges of the moving object to keep it from twisting or racking. It is usually not desirable or possible to provide such guidance because of physical restrictions or cost. Sometimes the framing of the system is not strong enough to provide adequate support.
The solution to all of these problems is to use some means of developing synchronized push/pull force at two points, mounted far enough apart to give a stable operation to the motion of the object. Traditionally there have been two methods of developing two point synchronized motion. 1. Use two screws of some sort that are operated together by a gear train or timing belt. 2. Use two rack and pinion systems connected together by a common shaft. Both systems require an electric motor to provide rotation and both are expensive. In the past all attempts to use air or hydraulic means to provide two point force to move an object has failed because the cylinders do not stay synchronized. Providing heavy guide bearing to force synchronization does not help and is counter to design and cost constraints.
Thus, an apparatus having the aforementioned advantages and solving the aforementioned problems is desired.